The most accurate way to check an infant’s temperature is with a digital rectal thermometer, especially for babies under 3 months old. A reading of 100.4°F (38°C) or higher is considered a fever at any age, but the urgency of that number depends on how old your baby is.
Which Thermometer to Use by Age
Not every thermometer works well for every age. Infant ear canals are too small to give reliable readings with a tympanic (ear) thermometer until at least 6 months of age, and forehead scanners have limitations in the youngest babies. Here’s how it breaks down:
- Birth to 3 months: A standard digital thermometer used rectally is the gold standard. Recent research suggests temporal artery (forehead) thermometers may also provide accurate readings in newborns, but rectal remains the most trusted method at this age.
- 3 to 6 months: You can use a rectal, armpit (axillary), or forehead thermometer. Ear thermometers are still not recommended.
- 6 months and older: All methods are on the table, including ear thermometers. Rectal readings remain the most accurate, but forehead and ear thermometers are faster and easier.
If you’re buying one thermometer to cover the newborn period, get a basic digital thermometer. They cost under $10 and give the most reliable reading for the age group where accuracy matters most.
How to Take a Rectal Temperature
Rectal temperatures sound intimidating, but the process takes under a minute and is painless for your baby. You’ll need a digital thermometer (label it “rectal only” so it never gets used orally), petroleum jelly, and a calm setup.
Lay your baby belly-down on your lap or on a firm, flat surface. Some parents prefer placing the baby face-up and holding the legs up toward the chest, similar to a diaper change position. Either works. Apply a small dab of petroleum jelly to the tip of the thermometer. Gently insert the tip about half an inch to one inch into the rectum. Don’t force it. Hold the thermometer in place with two fingers, keeping a gentle grip on your baby so they don’t roll or wiggle it deeper. Wait for the beep, then slide it out and read the number.
Clean the thermometer afterward with rubbing alcohol or soap and warm water, and let it dry completely before storing. Keep this thermometer separate from any you use for oral or armpit readings.
Using a Forehead or Armpit Thermometer
Forehead (temporal artery) thermometers work by scanning the blood vessel that runs across the forehead. You swipe or hold the device across the skin and get a reading in a few seconds. These are convenient and non-invasive, and they’re considered reliable for babies 3 months and older. Some newer evidence supports their use in newborns too, though rectal is still preferred when precision matters most.
Armpit readings are the least accurate of the common methods. The temperature under the arm runs about 1°F lower than a true core body temperature, so an armpit reading can miss a low-grade fever. If you take an armpit reading and it seems borderline, follow up with a rectal reading to confirm. To get the best armpit result, tuck the thermometer tip snugly into the center of your baby’s armpit, hold their arm down against their body, and wait for the beep.
What Counts as a Fever
A rectal temperature of 100.4°F (38°C) or higher is a fever. That single number applies across all ages, but what you should do about it changes significantly depending on how old your baby is.
For infants under 3 months, a fever of 100.4°F or higher needs immediate medical attention. At this age, even a low-grade fever can signal a serious infection because the immune system is still immature. Don’t wait to see if it comes down on its own.
For babies 3 to 6 months old, the threshold for seeking medical advice is 102°F (38.9°C) or higher. For babies over 6 months, the guideline shifts to 103°F (39.4°C) or higher. These thresholds assume your baby is otherwise acting relatively normal. Any fever paired with concerning symptoms warrants a call regardless of the number on the thermometer.
Signs That Need Attention Beyond the Number
A thermometer gives you one data point. How your baby is acting tells you even more. Pay attention to these warning signs, especially alongside a fever:
- Unusual sleepiness: Sleeping far more than normal, being difficult to wake up, or seeming floppy and unresponsive.
- Dehydration: Fewer wet diapers than usual, crying without tears, a dry mouth, or a sunken soft spot on the head.
- Rash: Particularly one that appears quickly, blisters, or looks infected.
- Forceful vomiting: Not the normal spit-up, but vomit that shoots out. Especially concerning if your baby can’t keep liquids down for eight hours.
- Inconsolable crying: Fussiness that’s clearly more intense than usual and won’t settle with normal comfort measures.
Tips for Getting an Accurate Reading
Timing matters. If your baby was just bundled in heavy clothing, in a warm car seat, or recently had a bath, wait 15 to 20 minutes before checking. Overdressing alone can raise a baby’s body temperature enough to mimic a low-grade fever. Similarly, taking a reading right after breastfeeding or bottle-feeding can skew the result slightly.
If you get a reading that surprises you, whether high or low, take it again. Digital thermometers can occasionally give an off reading if they aren’t positioned correctly, especially with the armpit or forehead method. Two consistent readings a few minutes apart give you much more confidence than a single number. Write down the temperature, the time, and the method you used so you can report it accurately if you need to call your pediatrician.